As noted above, there are plenty of free focus charts, and plenty of gotchas as well.
I've found two major areas to be careful of.
1. Use a test method that absolutely ensures that you are focused correctly. just putting the AF spot on a number on a sloped ruler almost guarantees that it will focus differenly every time.
2. It is very difficult to see tiny differences in focus sharpness, and you can misjudge the proper setting easily.
Other things to watch out for are proper alignment to a target. If you are not perpendicular and level, thats going to give you some error.
One way to check to see if adjustment is reasonably achieved is to put the camera on a sturdy tripod, focus on the target, and then use live view and live AF to see if focus improves noticably. Live Autofocus is slow, but almost always perfect.